One of the most effective therapeutic methods for sleep apnea syndrome is a continuous positive airway pressure (CPAP) method. While in this therapy a respiratory equipment is adopted for supplying positive pressure gas at about 400 to 2000 Pa to the nasal cavity, the nasal cavity and the mouth or the face of a user during sleep, in recent years devices with a function to control a pressure automatically responding to an occurrence state of apnea during therapy have also been used. A nasal intermittent positive pressure ventilation (NIPPV) method is used as one of the most effective therapeutic methods for ventilatory insufficiency, in which a respiratory equipment is adopted for supplying intermittent positive pressure gas at about 400 to 2400 Pa to the nasal cavity of a user. A mask for supplying positive pressure gas while applying pressure to the nasal cavity, a mask for supplying positive pressure gas while covering the nasal cavity and the mouth, and a mask for supplying positive pressure gas while covering the full face are called a nasal respiratory mask system, a full face mask, and a total face mask, respectively.
A nasal respiratory mask system is described as an example in the following, but its content may be applied to a general respiratory mask system including a full face mask, a total full face mask and the like.
A nasal respiratory mask system is, for example, as shown in FIG. 1, generally provided with a hollow nasal mask cushion 4 tightly sealing the face of a user, a frame 3 retaining the nasal mask cushion 4 at a predetermined position for supplying respiratory gas under positive pressure, a headgear 16 connected with this frame 3 and elastic headgear straps 17 and 18 to link this headgear 16 with the frame 3 in order to continuously supply positive pressure to the nasal cavity of a user (to both nose and mouth in case of a full face mask and to a full face in case of a total face mask). Such a nasal respiratory mask system then tightly seals the face of a user with the nasal mask cushion 4 by the tension of headgear straps 17 and 18 (see, for example, Patent Document 1, Patent Document 2 and Patent Document 3).
However, it is known that various factors cause slipping of the nasal mask cushion during wearing even if a nasal respiratory mask system is once worn properly. Accordingly, measures responding to each cause are taken to prevent or reduce its slip. For example, in a nasal respiratory mask system disclosed in Patent Documents 1 and 2, a bellow is mounted between a frame section and a face contact section of a nasal mask cushion to have no effect on the nasal mask cushion by slipping the frame section caused by contact with bedclothes. In a nasal respiratory mask system described in Patent Document 4, a flexible material is used in a wall around a connection section with a hose for supplying positive pressure gas in order to reduce slipping of a frame section pulled by the hose for positive pressure gas connected with the frame section.    Patent Document 1: Japan Patent Laid-Open Publication No. H11-000397.    Patent Document 2: WO 01/097893    Patent Document 3: WO 98/04310    Patent Document 4: WO 96/17643